Both time and communication overhead are required to locate and deliver information to users who move from place to place in personal communication services (PCS) systems. Such users are referred to as nomadic users.
Systems with nomadic users differ from cellular telephone systems in that as nomadic end users move, the communication services provided to them also move, but their terminal equipment need not. In particular,
1. Nomadic users are not restricted to accessing the communications network via wireless technology; they may use either wireline or wireless communication access technology as convenient.
2. Nomadic users may use different terminal equipment (or communication modes) at different locations in the system, including wireline fixed telephones, facsimile machines, pagers, laptop computers, etc., as well as cellular telephone devices.
3. The communications services available to nomadic users move with them, within the limitations of the communication mode chosen. For example, if a user subscribes to an information service which delivers both text and graphics, the text would be available as the user moves, but the graphics may not be available on some communication modes, e.g. pagers.
The combination of flexible communication modes and access technologies, and portable communications services, distinguishes systems with nomadic users from mobile cellular users.
Nomadic users receive calls via either wireless access or wired access through the Local Exchange Carrier (LEC) wireline network. If wireless access is used, it may be obtained through PCS Service Providers, which may subscribe to LEC PCS Access Services. In general, calls may deliver voice, data, text, facsimile or video information.
A location area or registration area is the location of a nomadic user as known by the wireline network. For users attached directly to a wireline network, the registration area is defined as the point of attachment. For users attached via wireless links, the situation is described as follows. In order to deliver calls by wireless links, the geographical region covered by a PCS network is divided into cells. Each cell is primarily served by one radio base station, although a base station may serve one or more cells. The base station locates a user and delivers calls to and from the user, by means of paging within the cell(s) it serves. Base stations are connected to the rest of the wireline network by wireline links. Cells are aggregated into larger geographical regions; from the point of view of the wireline network, the registration area of a wireless user is defined by the collection of cells in which the user is located. The size of the geographical region corresponding to a registration area may depend upon several factors including population density and PCS market penetration.
One type of PCS system is based on IS-41 or GSM standards and utilizes a signalling network similar to Signalling System 7 network. Studies have shown that, with predicted penetration levels of PCS in telephone exchange calling regions, there will be significant loads upon the signalling network and network databases and that these loads are dependent upon the data management strategies adopted.
The basic strategy for locating nomadic users defined in the IS-41 standards document for North American cellular systems and the Bellcore PCS Network and Operations Plan is a two-level strategy. In a two-level strategy, each user is assigned to with a specific database in the network, called its Home Location Register (HLR) which contains the user's profile, security information, and other data, as well as location information. Each registration area is served by a base station connected to a Mobile Switching Center (MSC), and each MSC, or set of MSC's, is served by a database called the Visited Location Register (VLR).
When a user u moves from registration areas i to j corresponding to MSCs, the MSC at j informs its VLR of user u's presence. If i and j are served by the same VLR, it records the new MSC serving u and takes no further action. Otherwise, the VLR decodes u's identification number and informs u's HLR. The HLR updates its information about u's location (registration) and also informs the VLR serving i, which deletes information about u (deregistration).
When a user u at registration area i attempts to locate user v and v is visiting registration area j, the MSC at i queries v's HLR, which in turn queries v's VLR. The VLR obtains specific location information from MSC j and returns it to the HLR, which in turn returns it to i; this information is sufficient to complete routing of the call.